In Florida, strawberry, beans, squash and other crops risk significant damage from extended freezes, but most eyes are on the billion-dollar-a-year citrus industry.

The National Weather service has forecast temperatures plunging below the zero mark from the Great Plains to the Northeast, and into the 20s in Louisiana, Mississippi, Alabama, Georgia and Florida, where it has issued hard freeze watches.

It is one of the longest cold snaps in decades, and lows in some parts of Florida tonight are set to break records.

Watch More on the Cold Weather Tonight on ABC's 'World News with Diane Sawyer.'

Bob Norberg, the chief economist for the Florida Department of Citrus said, "From the forecast that I've seen, we're right on the edge of crop damaging weather versus not having an impact."

The $400 million-a-year strawberry industry could also be vulnerable.

Temperatures under 28 degrees sustained for over four hours can cause significant crop damage, said Norberg. The nearly week-long cold spell is the longest endured in Florida since 2001, according to the National Weather Service.

Kay Subick, scampering from plant to plant at her Lucas Nursery in Oviedo, Fla., said she's sold out of thermal blankets designed to cover plants. She'd taken as many of her own plants as she could indoors and left the rest to the elements. She thinks several types of plants and highly cultivated gardens in the state would likely suffer, along with the crops.

"So we're taking inside as much as we possibly can. We're covering, we are also spraying with frost protection."

Sleep-deprived farmers are doing the same, cranking open their irrigation systems during the coldest part of night -- the pre-dawn hours. The developing mist and the formation of ice generate heat, which can insulate large swaths of orange groves. The frozen condensation on fruit and blossoms does the same, encasing the fruit in what farmers like to call a miniature igloo, keeping the fruit inside above 28 degrees -- warm enough to survive.

Sunrise brings relief, as will warmer temperatures later in the week. But weekend temperatures are again forecast to dive into the 30s.

A disrupted supply of orange crops may raise prices. Norberg describes orange juice prices as "semi-elastic." For instance, a 10 percent decline in product would trigger a price hike of between 5 percent to 7 percent. For the consumer, he explains, that would mean a two- to three-cent rise in price for an eight-ounce glass of OJ.

After climbing all last year, orange juice futures tumbled just before the new year on speculation that this cold snap would not affect the citrus harvest.

Across much of the U.S. the cold led to strong demand for natural gas and heavier than average foot traffic at hardware stores, like Home Depot.

In Texas, people are bundling against the cold. In Houston, the National Weather Service issued a hard freeze warning for 10 counties in and around Houston. Newcomer Jennifer Sloan, with the wind whipping against her track jacket, told ABC's KTKR-TV, "We just moved from Phoenix, Ariz., and we were surprised. We thought the the weather was parallel to Phoenix and its not!"

Charleston, S.C., expected subfreezing temperatures all week. Parts of West Virginia could see 4 to 8 inches of snow by Wednesday, and many counties canceled school Tuesday, The Associated Press reported.

In Nashville, Tenn., the overnight low was 12 degrees, An 81-year-old man with Alzheimer's disease reportedly wandered outside in his bathrobe and froze to death.